Electron microscopes are generally used to investigate the ultrastructure of biological samples such as cells and tissue, polymer resin samples, and crystalline samples such as inorganic substances. Two types of electron microscopes are known, including scanning electron microscopes (hereinafter sometimes referred to as SEMs) and transmission electron microscopes (hereinafter sometimes referred to as TEMs).
Patent Literature 1 describes a sample preparation method in which a sample to be observed is embedded in an embedding medium and thin sectioned with a microtome etc. The embedding medium used is an epoxy resin, an unsaturated polyester resin, an acrylic resin, or the like, all of which are insulating materials. Consequently, the resin tends to charge at the time of observation with an electron microscope and this may cause difficulty in examining the subject. In some cases such charging may result in discharge phenomena inside the electron microscope, which may damage the detectors. These problems become obstacles especially when insulating materials are observed with a SEM.
In order to overcome the problems relating to charging during observation with a SEM, various modifications of observation technique have been conventionally introduced. For example, observation with a SEM has been conventionally carried out under conditions in which incident electrons and emitted electrons are in a particular angle relationship, or under low-vacuum conditions, or under low-current conditions. However, these modifications have drawbacks. For example, the observation range is narrow, contrast and resolution are insufficient, and elemental analysis cannot be conducted.
Patent Literature 2 discloses an epoxy resin composition having antistatic performance, produced by adding an alkali metal salt and a polyether-based polymer to an epoxy resin. However, the composition lacks sufficient mechanical strength and therefore does not provide satisfactory performance as an embedding medium, including embedding performance and sectioning quality.